Electrical & Electronics
Voltage Divider Calculator
Calculate output voltage with load and tolerance, or find standard resistor pairs for a target voltage.
Your inputs
Your results
- Output voltage
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- Unloaded output voltage
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- Supply current
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- Output resistance, unloaded
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- Minimum · Vout
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- Maximum · Vout
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| R1 · Power | — |
|---|---|
| R2 · Power | — |
| Load current | — |
Vout = Vs × R2 / (R1 + R2)Best resistor pairs
| R1 | R2 | Vout | Target error · % | Use pair |
|---|
Calculate a voltage divider with loading and tolerance
Calculate the output of a resistive divider, optionally including a resistive load and resistor tolerance. R1 connects the supply to the output; R2 connects the output to ground. The search mode compares nearby preferred resistor pairs for a target voltage. This is a fixed DC model, not a voltage regulator or a model of changing load current.
Step by step
- Choose output calculation and enter supply voltage, top resistance R1 and bottom resistance R2. The resistance fields use kΩ unless you type a recognized unit suffix.
- Enable the load option if a resistance is connected from output to ground, then enter its value. Enter the same tolerance percentage for both divider resistors and review nominal output, bounds, current and dissipation.
- To find a pair, switch to search mode, enter a target below the supply, and choose a resistance scale and E-series. Compare the listed candidates; select a pair to return to output mode and inspect it in detail.
Settings and limits
- Unloaded and loaded output
- Without a load, Vout = Vs × R2 / (R1 + R2). With a load RL, replace R2 in that ratio with R2 × RL / (R2 + RL). The displayed unloaded output is a useful comparison; the loaded result assumes RL stays constant.
- Tolerance bounds
- The minimum uses a higher R1 and lower R2; the maximum uses a lower R1 and higher R2. Supply and load resistance stay fixed. These are resistor-tolerance bounds rather than statistical predictions, and they exclude supply or load variation.
- Search and output resistance
- Search checks preferred values from one tenth to ten times the chosen scale and lists up to five pairs ranked by target-voltage error. It is a bounded search, not a search over every possible resistor. Displayed output resistance is R1 in parallel with R2, excluding the external load.
Worked example
In output mode, enter 12 V, R1 = 10 kΩ and R2 = 10 kΩ with 1% tolerance. The unloaded output is 6 V. Enable a 10 kΩ load across R2 and compare the changed results.
| RL | Vout | I | IL |
|---|---|---|---|
| ∞ | 6 V | 0.6 mA | 0 mA |
| 10 kΩ | 4 V | 0.8 mA | 0.4 mA |
The loaded lower resistance is 5 kΩ, so nominal output falls to 4 V. Supply current is 0.8 mA and load current is 0.4 mA. The tolerance bounds are approximately 3.96013 V and 4.04013 V; divider output resistance remains 5 kΩ.
Questions and troubleshooting
Why does connecting a load lower the output?
The load is in parallel with the bottom resistor, reducing the effective lower resistance and changing the divider ratio. Compare loaded and unloaded results using a realistic load resistance. A varying or non-resistive load needs a different model.
Can the divider supply a device at the target voltage?
The calculation shows how a specified resistive load affects the output; it does not hold voltage constant as a device's current changes. Inspect loading, current and resistor power. Use an appropriate regulated supply when the load requires voltage regulation.
Why are results blank in search mode?
The target must be positive and below the supply, resistances must be positive, and the scale must be within the supported range. Tolerance must also be valid. Check the highlighted field, select another scale or reset the calculator.